1. Field of the Invention
The present invention relates to an airgap type etalon as a Fabry-Perot interferometer, and more particularly to an airgap type etalon capable of freely adjusting temperature dependency of a wavelength characteristic (hereinafter called wavelength-temperature characteristic) and an apparatus utilizing such an etalon.
2. Related Art
Recently, increase of a transmission capacity in optical communications has resulted in adoption of a wavelength division multiplexing (WDM) transmission method. In such a WDM transmission method, wavelengths of different channels have been brought closer to one another for further increase of a transmission capacity. Correct transmission and reception of signal light including channels having such mutually closer wavelengths requires that a wavelength characteristic of an optical filter such as an etalon to be used in a transmission apparatus is stable relative to a change of environmental conditions such as temperature. As such, there is demanded an optical filter having an extremely low wavelength-temperature characteristic.
Further, in long-distance optical transmissions, an erbium (Er)-doped optical fiber amplifier (hereinafter called xe2x80x9cEDFAxe2x80x9d), for example, has been widely adopted as an optical repeater so as to increase a transmission distance. A gain characteristic of such an EDFA generally has a wavelength characteristic such as based on a composition of an optical fiber and the like. Thus, a technique for flattening such a gain wavelength characteristic of an EDFA has been put to practical use by utilizing, as a gain-equalizer (hereinafter called xe2x80x9cGEQxe2x80x9d), an optical filter such as an etalon having a loss wavelength characteristic opposite to the gain wavelength characteristic of the EDFA.
However, since the gain wavelength characteristic of the EDFA is also changed according to the temperature of an erbium-doped optical fiber (EOF), this characteristic is affected by the change of the environmental temperature. As such, deviations of optical SNR""s (signal/noise ratios) of optical signals at respective wavelengths have been problematically caused in a WDM transmission apparatus. To deal with this problem, there is desired a gain-equalizer capable of passively flattening the gain wavelength characteristic of the EDFA even when this characteristic is changed due to a temperature change. Unfortunately, realization of such a gain-equalizer requires an optical filter having a higher wavelength-temperature characteristic, contrary to the aforementioned demand for an optical filter.
Further, EDFA""s are being widely used in a superlong-distance transmission for undersea repeating. In such usage, a change of an input level of an EDFA due to repair of an optical fiber transmission path and/or a change with time lapse has problematically resulted in a change of the gain of the EDFA when the output level of the EDFA is fixedly controlled. Since the gain change of the EDFA causes an inclination in the wavelength characteristics of the EDFA, there is desired a wavelength characteristic varying apparatus capable of compensating for the inclination of the wavelength characteristic. However, realization of such a wavelength characteristic varying apparatus requires two kinds of optical filters such as having high wavelength-temperature characteristics, respectively, and the respective wavelength temperature characteristics having shift directions opposite to each other.
Concerning such optical filters, various demands are presented not only for transmission wavelength characteristics of the filters but also for the temperature dependencies of such characteristics, corresponding to usage and purpose of the filters. Therefore, such optical filters are needed to be designed in response to these demands. As the aforementioned optical filters, there have been generally used: an etalon (Fabry-Perot interferometer) of a solid type or airgap type; a multilayered dielectric film filter; a fiber Bragg grating (FBG); and the like.
Concrete constitutions of conventional etalons include those such as known from Japanese Unexamined Patent Publication Nos. 7-86673, 3-185402, 7-27943, and 9-257567.
Concerning a multilayered dielectric film filter or an FBG among the aforementioned optical filters, however, materials usable in the manufacturing process are limited to silica (SiO2) and the like, and wavelength-temperature characteristic is determined corresponding to the physical properties (an expansion coefficient; and a temperature coefficient of a refractive index), resulting in a defect that the degree of design freedom is extremely low.
Further, concerning a solid type etalon, although various kinds of transparent multicomponent glass materials can be used, the wavelength-temperature characteristic of the etalon is also determined corresponding to the physical properties (an expansion coefficient; and a temperature coefficient of a refractive index) of the etalons, resulting in a limited degree of design freedom. In this respect, synthetic resins such as acrylic resins and polycarbonate resins are conceivable as transparent materials. However, it is difficult to adopt such synthetic resins as materials of etalons, due to the properties of these resins such as: water absorptivity leading to susceptibility to humidity; poor flatness even after machining; and lower light resistance leading to occurrence of material deterioration upon entrance of light at a higher intensity.
Concerning an airgap type etalon, there has been conventionally used a constitution in which a gap material is interposed between two transparent parallel flat plates such as glass as shown in FIG. 21. Each of the transparent parallel flat plates is formed with an antireflection coating on an outer flat surface and a reflection augmenting coating on an inner flat surface. In such a constitution, the gap material can be provided not only by a transparent material but also by an opaque material, leading to a slightly higher degree of freedom for selecting a material as compared to a solid type etalon. However, gap materials are actually restricted to a glass material and/or some metals in view of stability for the environment and machinability into a flat surface. Accordingly, in the present state, a sufficient degree of design freedom for wavelength-temperature characteristic is hardly obtained.
As described above, it has been rather difficult to actually obtain a desired wavelength-temperature characteristic in conventional optical filters. Also, since suitable optical filters have not been obtained in various devices such as wavelength detecting devices, gain-equalizers, and wavelength characteristic varying devices, which devices need optical filters having predetermined wavelength-temperature characteristics, there has been a problem in that a desired performance can not be achieved.
There will be now concretely explained problems of conventional etalons described in the aforementioned publications.
Firstly, the etalon described in Japanese Unexamined Patent Publication No. 7-86673 includes a gap material constituted by mutually joining a material having a positive expansion coefficient and another material having a negative expansion coefficient, for the purpose of eliminating a temperature dependent change of a gap length of the airgap type etalon to thereby eliminate wavelength-temperature characteristic. In such a constitution, it is possible to realize an etalon having a lower wavelength-temperature characteristic, but it will be difficult to also provide an etalon having a higher wavelength-temperature characteristic. Further, in constituting an etalon utilizing this conventional technique, it is required to adopt a specific ceramic material such as having a negative expansion coefficient, leading to a defect that the degree of design freedom for wavelength-temperature characteristics is restricted. Moreover, the ratio of thickness between two gap materials is determined by the expansion coefficients of respective materials. Thus, in setting a larger free spectral range (hereinafter called xe2x80x9cFSRxe2x80x9d) and a shorter gap length, a thin gap material is required. However, realization of such a thin gap material is predicted to be difficult to manufacture, resulting in a problem of an impractical gap material.
Next, the etalon described in Japanese Unexamined Patent Publication No. 3-185402 or 9-257567 has a constitution in which a material having an expansion at coefficient different from that of a solid type etalon is joined to the periphery of the solid type etalon, so that the material acts to apply a thermal strain and a thermal stress to the etalon upon temperature change, to thereby eventually maintain the optical thickness (coherence length) of the etalon to be constant. Even in such a constitution, however, it is difficult to realize an etalon having a higher wavelength-temperature characteristic, and there is even such a possibility of breakage of the etalon due to the positive application of the thermal strain and thermal stress.
Meanwhile, the etalon described and established in Japanese Unexamined Patent Publication No. 7-27943 has a constitution in which a solid type etalon and an airgap type etalon are combined with each other. If the temperature rises, the optical thickness (coherence length) of the solid type etalon is increased due to the thermal expansion and the increase of refractive index thereof while the optical thickness of the airgap type etalon is decreased so as to offset such an increased optical thickness, to thereby eventually maintain the total optical thickness of the etalon to be constant. Even in such a constitution, however, it is difficult to realize an etalon having a higher wavelength-temperature characteristic, and there is such a defect that the degree of design freedom of the wavelength-temperature characteristic is restricted as described above due to the partial inclusion of the solid type etalon.
The present invention has been carried out in view of the conventional problems as described above, and it is therefore an object of the present invention to provide an airgap type etalon having a higher degree of design freedom of a wavelength-temperature characteristic so that such a wavelength-temperature characteristic can be freely adjusted, and to provide an apparatus utilizing such an etalon.
To achieve the above object, an airgap type etalon according to the present invention comprises: a fixing member having at least one flat surface; a first parallel member, which is transparent to incident light and has parallel flat surfaces, one of the parallel flat surfaces thereof being joined to the flat surface of the fixing member; at least one second parallel member, which has parallel flat surfaces in which a distance between the parallel flat surfaces thereof is greater than a distance between the parallel flat surfaces of the first parallel member, and has an expansion coefficient different from that of the first parallel member, one of the flat surfaces of the second parallel member being joined to the flat surface of the fixing member so as to surround the outer periphery of the first parallel member; and a transparent member, which is transparent to incident light and has opposite flat surfaces, one of the flat surfaces thereof being joined to the other flat surface of the second parallel member opposite to the joined surface to the fixing member; wherein a Fabry-Perot interferometer is formed based on an airgap positioned between the flat surface of the first parallel member and the flat surface of the transparent member facing each other.
According to such a constitution, the light input into the airgap type etalon of the present invention is reflected multiple times between the parallel flat surface of the first parallel member and the flat surface of the transparent member facing each other, to thereby generate Fabry-Perot interference. The length of the airgap formed between the reflecting surface of the first parallel member and the reflecting surface of the transparent member is to be determined by a difference in thickness between the first parallel member and the second parallel member (i.e., by a distance between the parallel flat surfaces), on the basis of the flat surface of the fixing member. In this way, it becomes possible to use the thicknesses and expansion coefficients of the first and second parallel members as design parameters even when the optical distance of the airgap has been determined corresponding to a required optical characteristic. By suitably setting these design parameters, it becomes possible to widely adjust the temperature dependency of the transmission wavelength characteristic of the etalon.
A concrete constitution of the airgap type etalon may be such that the fixing member has a through-hole for passing light therethrough, the first parallel member is formed with an antireflection coating on one flat surface thereof, and this flat surface formed with the antireflection coating is joined to the flat surface of the fixing member around the through-hole, and the transparent member is formed with an antireflection coating on one flat surface of the transparent member opposite to the joined surface of the transparent member to the second parallel member.
Alternatively, the constitution may be such that the fixing member is transparent to incident light, and is formed with an antireflection coating on a surface opposite to the flat surface thereof, and the transparent member is formed with an antireflection coating on the other flat surface thereof opposite to the joined surface of the transparent member to the second parallel member.
Further, concerning the airgap type etalon, reflection augmenting coatings may be formed on the flat surfaces of the first parallel member and the transparent member facing each other, respectively.
According to such a constitution, there can be obtained a required maximum loss by forming the reflection augmenting coatings on the flat surfaces of the first parallel member and the transparent member, respectively, in case of insufficient Fresnel reflection at each of the flat surfaces of the first parallel member and transparent member facing each other.
A wavelength detecting apparatus according to the present invention comprises: a first branching portion and a second branching portion for extracting branched light from a main light path, respectively; an optical filter for transmitting the branched light from the first branching portion and for giving a wavelength characteristic to the thus transmitted light; a first light receiving portion for converting the transmitted light from the optical filter into an electrical signal; and a second light receiving portion for converting the branched light from the second branching portion into an electrical signal, wherein the optical filter is constituted by employing the airgap type etalon of the present invention, and wherein the thicknesses and expansion coefficients of the first and second parallel members are set so that the temperature dependency of the transmission wavelength characteristic becomes 1 pm/xc2x0 C. or less.
According to such a constitution, the first light receiving portion generates an electrical signal having a wavelength characteristic corresponding to the characteristic of the optical filter, and the wavelength of light input into the wavelength detecting apparatus is detected based on the electrical signal obtained by the first light receiving portion and a reference electrical signal obtained by the second light receiving portion. This wavelength detection is performed based on the optical characteristic of the airgap type etalon having a small temperature dependency of the transmission wavelength characteristic, so that the temperature dependency of the detected wavelength is reduced. This type of wavelength detecting apparatus can be utilized as a so-called wavelength locker, for example, by being combined with a semiconductor laser diode.
A gain-equalizer according to the present invention comprises: a plurality of optical filters having periodical transmission wavelength characteristics shifted from one another by an approximately xc2xd cycle at a predetermined temperature, in which the respective transmission wavelength characteristics have mutually different temperature dependencies such that a transmission wavelength characteristic obtained by synthesizing the transmission wavelength characteristics of the plurality of optical filters is passively varied corresponding to a temperature change, wherein at least one of the plurality of optical filters is constituted by employing the airgap type etalon of the present invention, and the thicknesses and expansion coefficients of the first and second parallel members are set so that the temperature dependency of the transmission wavelength characteristic of the airgap type etalon becomes 25 pm/xc2x0 C. or more.
According to such a constitution, there can be performed gain-equalization following a temperature change, in accordance with the transmission wavelength characteristic obtained by synthesizing the transmission wavelength characteristics of the plurality of optical filters including the airgap type etalon having a large temperature dependency of the transmission wavelength characteristic. This type of apparatus is suitable, for example, as compensation means for flattening a wavelength characteristic of an optical signal output from an optical amplifier having a larger temperature dependency of gain wavelength characteristic.
A wavelength characteristic varying apparatus according to the present invention comprises: a plurality of optical filters having periodical transmission wavelength characteristics, in which the transmission wavelength characteristics have mutually different temperature dependencies; and temperature controlling means for controlling the temperature of the plurality of optical filters such that an inclination amount of a transmission wavelength characteristic obtained by synthesizing the transmission wavelength characteristics of the plurality of optical filters can be positively varied by a temperature control by the temperature controlling means, wherein the plurality of optical filters are constituted by employing at least two airgap type etalons of the present invention, and the thicknesses and expansion coefficients of the first and second parallel members are set so that the temperature dependency of each of the transmission wavelength characteristics becomes 25 pm/xc2x0 C. or more and the shift directions of temperature dependencies of transmission wavelength characteristics of the airgap type etalons are opposite to each other.
According to such a constitution, since the plurality of optical filters are constituted by employing the airgap type etalons having large temperature dependencies of transmission wavelength characteristics in which the shift directions of temperature dependencies of transmission wavelength characteristics are opposite to each other, the transmission wavelength characteristics of the airgap type etalons are shifted in the directions opposite to each other when the temperatures of the optical filters is changed by the temperature controlling means. In this way, it becomes possible to vary the inclination amount of the transmission wavelength characteristic obtained by synthesizing the transmission wavelength characteristics of the plurality of optical filters by the temperature control. This type of apparatus is suitable, for example, as compensation means for flattening a wavelength characteristic of an optical signal output from an optical amplifier having a gain wavelength characteristic which varies corresponding to an operating condition.
Other objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments when read in conjunction with the accompanying drawings.